In recent years, a sleeker and more lightweight train has become popular, notably the turbine-powered train. The turbine-powered train is conceived on aerodynamic principles and is powered by an aircraft-type gas turbine engine, making it lighter, faster and quieter than conventional trains.
In a modern gas turbine engine, the engine produces its own pressurized gas, and does this by burning a fuel. The heat that comes from burning the fuel expands air, and the high-speed rush of this hot air spins the turbine. A simple gas turbine engine has a compressor, a combustor and a turbine. A rotating compressor draws in air from the atmosphere, pressurizes it, and forces it into the combustor (i.e. the furnace) in a steady flow. Fuel, such as oil, natural gas or pulverized coal, is forced into the air and burns, raising the temperature of the mixture of air and combustion products, thereby increasing the heat energy. This high-energy mixture then flows through the turbine, dropping in pressure and temperature as it drives the moving blades and develops mechanical energy. The spent gases then leave at atmospheric pressure but at high temperature. The turbine drives the compressor rotor through a shaft and also an external load through the load coupling.
Train accessories may be driven mechanically by an accessory drive shaft and gearbox connected directly to the turbine shaft. The mechanical connection from the turbine shaft may be through an engine-mounted gearbox or through a power takeoff shaft to a remotely mounted gearbox. Examples of such train accessories include: tachometers, generators (alternators), hydraulic pumps, fuel pumps, oil pumps, fuel controls, starters, etc.
Typically, the turbine engine turns a high-speed generator, producing electricity. Power from the generator is then provided to one or more variable-speed electric motors, which drive the wheels of the train. When the train is in motion, various different unpredictable situations may cause load transients to be experienced by the turbine engine, for example wheel slip or emergency breaking.
As is well known, in cases where the turbine engine sees its load decrease significantly, the turbine engine will have a tendency to speed up and may reach speeds in excess of the rated maximum speed for the engine (commonly referred to as overspeed mode). This overspeed mode is undesirable since it can be damaging to the turbine engine, which operates most efficiently for a constant rather than a fluctuating load.
In existing turbine-powered trains, a common solution to a detected overspeed mode is to shut-off the turbine engine, after which the turbine engine has to be re-started. Unfortunately, repeated shutting off and re-starting of a turbine engine inevitably causes wear to the turbine engine, thus reducing the active lifetime of the turbine engine. Furthermore, since it can take several seconds to re-start a turbine engine, having to re-start the turbine engine each time a severe wheel slip occurs may prove to be quite impractical, especially when the track conditions are such that such wheel slips occur repeatedly and at short intervals.
In another solution, the fuel delivery to the turbine engine is controlled, whereby this fuel delivery is decreased or completely halted upon detection of a significant load decrease, for preventing over-revving of the turbine engine. Unfortunately, because of the risk of surge of the turbine compressor, a time lag must exist between detection of the load transient and the decrease in fuel delivery to the turbine engine, and the turbine engine may still increase in speed during this time lag. Furthermore, controlling the fuel delivery to the turbine engine does not prevent having to repeatedly decrease and increase the speed of the turbine engine, which will inevitably cause more wear to the turbine engine than maintaining the turbine engine at a constant speed.
In light of the foregoing, a need clearly exists in the industry for an improved system and method for controlling the speed of a turbine gas engine during load transients.